The present invention relates to a friction stir welding method that is particularly preferable for joining aluminum alloy members to form transportation devices such as railway cars, or industrial equipment, science-related equipment, electric appliances, and so on.
Friction stir welding is a method performed by inserting a rotating shaft (called a rotary tool) to the joint portion between members to be welded and moving the rotary tool along the joint line, thereby heating, softening, plasticizing and solid-phase welding the joint portion. The rotary tool comprises a large-diameter portion and a small-diameter portion. During welding, the small-diameter portion is inserted to the member(s) to be welded, and the end surface of the large-diameter portion comes into contact with the member(s). A screw thread is formed to the small-diameter portion. The end surface of the large-diameter portion facing the small-diameter portion is sloped and concaved. The central axis of the rotary tool is tilted along the direction of movement of the rotary tool. That is, the axis of the tool is tilted rearward to the direction of movement thereof.
According to another example, projections protruding to the side from which the rotary tool is inserted are formed to the two members to be welded, and friction stir welding is performed by inserting the small-diameter portion of the rotary tool to the abutted portion and inserting the large-diameter portion of the rotary tool to the projections. The metal material constituting the projections is used as the source material to fill the gap formed between the two abutted members. The projection can be formed only to one of the two members to be welded. Such method is used to weld extruded members, the friction stir welding being performed to the portion where the extruded direction of one member is orthogonal to that of the other member.
Moreover, when performing friction stir welding to form a car body of a railway car and the like having windows provided thereto, a plural number of rotary tools are equipped to one traveling body for the welding process. The traveling body is stopped just before the window portion, and all the rotary tools are retreated from the welded members, before reinserting the rotary tool(s) that does not have the window portion along its path. Then, the movement of the traveling body is restarted, and the friction stir welding is continued.
The above mentioned prior art methods are disclosed in Published Japanese Translation of PCT Patent Application No. 508073/97 (EP 0752926 B1), and Japanese Patent Publication Laid-Open No. 2000-343248 (EP 1057576 A2).
The side walls of the railway car body are provided with entrances for the crew etc. to get on and off the train. Since a great load is applied to the entrance, a thick rim member is welded onto the panels constituting the side walls of the car body. The rim member comprises of extruded members. The panels and the rim member of the car body are joined by friction stir welding. The rotary tool used for the friction stir welding is moved along the rim member.
In general, the rim member is formed by bending an extruded member into a U-shape. The bent corners are curved in arc-like shapes.
We will now explain how to form the rim member by welding three extruded members, the left block, the right block, and the upper block. The blocks are substantially orthogonal to one another. In the present case, the joint between the left block and the upper block and the joint between the upper block and the right block are substantially right-angled. The entrance through which the crews get on and off the train is generally equipped with such rim member.
When friction stir welding the rim member with the plate of the car body surrounding the rim member, the direction of the rotary tool (that is, the direction of movement of the tool) must be varied by 90 degrees at the joint (right-angled corner). This is because the rotary tool must be tilted along the direction of movement. The direction of the rotary tool is varied either by changing the position of the rotary tool or by changing the position of the members to be welded.
Upon varying the direction of the rotary tool, the movement of the tool must be stopped. Therefore, the movement of the rotary tool is stopped but the rotation of the tool is continued with the tool inserted to the joint of the members being welded. This causes defects to occur at the joint of the friction stir weld.
The same problem occurs even when the angle of the movement of the tool varies more than or less than 90 degrees. The direction of movement of the rotary tool is not just varied when welding a rim member to the entrance opening of the car body, but also when forming various members used in the field of construction, in the field of industrial appliances, or in other fields.
An optical sensor is used to detect the abutted portion for inserting the rotary tool thereto and guiding the movement of the tool, but when the direction of movement (the direction of the joint line) varies greatly (i.e., 90 degrees), the optical sensor fails to detect the abutted portion when it approaches the corner area.
Moreover, since the car body and the rim member of the entrance is formed of extruded members, and since the direction of extrusion is orthogonal to one another, the projection serving as the source material for filling the gap existing at the abutted portion can only be provided to one of the members to be welded. Therefore, defect is likely to occur to the joint.
The first object of the present invention is to provide a friction stir welding method capable of providing a good weld even when the angle of the joint line varies greatly.
The second object of the present invention is to provide a friction stir welding method capable of providing a good weld when bonding the abutted portion between extruded members.
The first object of the present invention is achieved by retreating the rotary tool from the members being welded when the tool moving along the joint line reaches the position in which the direction of the joint line changes, changing the direction of the rotary tool or the welded members, reinserting the rotary tool to the welded members, and moving the rotary tool along the new joint line.
Further, the object of the invention is achieved by abutting the end portion of a first member against the end portion of a second member, the abutted line or joint line varying greatly, wherein the relative movement of the rotary tool against the joint line is controlled based on a data stored in advance.
The second object of the present invention is achieved by abutting the end portion of a first member against the end portion of a second member having a projection formed thereto, performing fillet welding to the end portion of said second member along the projection, and performing friction stir welding by inserting a rotary tool to the abutted portion.